Oxycombustion chamber

ABSTRACT

The present invention relates to a combustion chamber comprising an enclosure ( 1 ) having at least one fuel injection means ( 2 ), at least one oxidizer injection means ( 3 ) and at least one combustion fumes withdrawal means ( 5 ), wherein enclosure ( 1 ) has the shape of a bent closed tube of any section, and the fuel ( 2 ) and oxidizer ( 3 ) injection means are arranged on enclosure ( 1 ) so as to be offset by an angle θ formed by each of the oxidizer and fuel injection positions with respect to centre (C) of the enclosure, ranging between 10° and 90°, and wherein the oxidizer injection means is a means of injecting an oxidant that is a gas with an oxygen concentration above 90%.

FIELD OF THE INVENTION

The present invention relates to the sphere of oxycombustion and moreparticularly to a combustion device whose geometry allows natural fumesrecirculation and is suited to the constraints linked with theoxycombustion of a liquid or gaseous feed.

The current economic activity and the increasing energy requirementsgenerate, through the use of fossil fuels, increasing emissions of CO₂,a greenhouse gas, to the atmosphere. These CO₂ emissions are suspectedof being the source of the climate change observed, and notably ofglobal warming.

A solution for reducing these emissions consists in capturing andsequestering the CO₂ emitted. However, the associated additional costsin terms of investment and the penalties on the overall efficiency ofthe equipments are for the time being prohibitive. To date, noeconomically satisfactory solution is available. Research is currentlybeing done in order to improve existing technologies, notably capturetechnologies.

Currently, the main CO₂ capture technologies studied are:

post-combustion capture, i.e. direct capture of the CO₂ present in aircombustion fumes. This capture requires adding a dedicated CO₂separation unit,

precombustion capture, i.e. prior CO₂ capture on the feed after a firststage of conversion to synthesis gas. This conversion generates aH₂-rich gas that, once freed of its carbon-containing compounds,releases no CO₂ during combustion,

oxycombustion capture: oxygen is substituted for air in the combustionstages. Fumes with a high CO₂ content are thus present at the combustionequipment outlet and they can then be directly sequestered withoutrequiring treatment in a CO₂ separation unit. On the other hand, adedicated oxygen production unit is required.

The present invention is concerned by the latter oxycombustiontechnique. In fact, this technique affords many advantages:

decreased amount of nitrogen oxide (NOx) formed (in the case of pure O₂,and in the absence of nitrogen compounds in the feed),

decreased amount of fumes generated under iso-power conditions,

decreased amount of thermal losses (corresponding to the “useless”heating of the nitrogen through the combustion cycle),

the concentration of the possible pollutants such as nitrogen oxide andsulfur oxide (NOx and SOx) being higher, separation is easier,

most components are condensable by compression and the condensation heatcan be advantageously used in the overall process scheme.

There still are many problems to be solved for oxycombustion. In fact,combustion in oxygen (O₂) causes markedly higher flame temperatures thatcan locally reach 2500° C. Generally, under O₂, a temperature increaseranging between 30% and 45% of the temperature obtained in air (in ° C.)in an adiabatic configuration is observed with commonly used fuels. Thisconstraint therefore requires specific technologies since suchtemperatures are not “manageable” in compact chambers with usualdesigns, whatever the alloys and refractories used.

One of the privileged solutions for reducing these hot spots consists inconducting an advanced burnt gas recirculation so as to dilute and tohomogenize the temperature profile around its mean value. This dilutionby inert gases and this swirling (the person skilled in the art speaksof entrainment ratio for quantifying the swirling intensity) thus avoidthe formation of hot spots responsible for local damage to the wallsand/or internals of combustion chambers. The recirculation needs areessentially required for reasons related to NOx formation and thermalefficiency. It can be noted that the recirculation needs are lessconstraining for combustion in air than for oxycombustion.

On top of being able to act upon the homogeneity of the combustion andof the temperatures, it is necessary to act upon the mean temperature bycooling the chamber so as to be in a temperature range allowing forexample to minimize the formation of pollutants while providing goodcombustion of the feed.

Some patents provide devices attempting to solve these problems posed byoxycombustion.

BACKGROUND OF THE INVENTION

Patent application US-2005/0,239,005 can notably be mentioned. Thispatent describes a specific burner that highlights the use ofrecirculation. It is a low NOx burner that can be used in a usualcombustion chamber such as atmospheric ovens with a wide space betweenthe burners and the internals (tubes, walls, etc.). This device thusrequires specific burners. The fuel and the oxidizer are injected inparallel, which disturbs recirculation and does not allow to provide acombustion distributed over the entire volume of the chamber.

Patent application WO-2004/065,848 relates to a device based on an ovalgeometry which, although it favours loop recirculation of the fumes soas to dilute the reactants, does not provide very good combustionhomogenization. Exchange is achieved with tubes running through thechamber, which may disturb recirculation.

U.S. Pat. No. 7,318,317 describes a turbine burner allowing looprecirculation of combustion gases in order to homogenize the combustion.In this device, the gases circulate only in a loop, which does not allowhomogeneous recirculation.

The object of the present invention thus is to overcome one or more ofthe drawbacks of the prior art by providing a specific and originallayout allowing to get round the constraints specific to oxycombustion.The combustion chamber thus has a geometry allowing naturalrecirculation of the fumes, suited to the constraints of oxycombustion.Furthermore, injection is carried out at various points so as to providerecirculation and dilution of the reactants, which allows to obtain acombustion distributed over the entire volume of the chamber withoutusing dedicated burners.

SUMMARY OF THE INVENTION

The present invention therefore provides a combustion chamber comprisingan enclosure having at least one fuel injection means, at least oneoxidizer injection means and at least one combustion fumes withdrawalmeans, wherein the enclosure has the shape of a bent closed tube of anysection, and the fuel and oxidizer injection means are arranged on theenclosure so as to be offset by an angle θ formed by each of theoxidizer and fuel injection positions with respect to the centre of theenclosure, ranging between 10° and 90°, and wherein the oxidizerinjection means is a means of injecting an oxidant that is a gas with anoxygen concentration above 90%.

According to an embodiment of the invention, the enclosure has the shapeof a tube bent in a closed circle.

In another embodiment of the invention, the enclosure has the shape of atube bent in an oval.

According to an embodiment of the invention, the section of the tube iscircular, oval or polygonal.

In an embodiment of the invention, the section of the tube istriangular.

In the combustion chamber according to the invention, the withdrawalmeans is arranged within the circle formed by the enclosure so as toachieve low-angle withdrawal.

According to an embodiment of the invention, the fuel injection meansconsist of at least one injection pipe arranged in the radial plane ofthe enclosure on the outside thereof.

In an embodiment of the invention, the fuel injection pipe forms anangle of inclination α formed by the longitudinal axis of the pipe andthe line passing through the fuel injection point and tangential to thetrajectory of the gas circulation after the injection point, said angleα ranging between 5° and 80°.

According to an embodiment of the invention, the fuel injection meansconsist of at least two low-angle pipes arranged in opposition on theenclosure, a first pipe allowing injection at the top of the enclosureand a second pipe allowing injection at the bottom of the enclosure.

In an embodiment of the invention, the fuel injection pipes form anangle of inclination α′ defined with respect to the longitudinal axis ofthe pipe and the radial plane of the enclosure, ranging between 5° and80°.

According to an embodiment of the invention, the oxidizer injectionmeans consist of at least one injection pipe arranged in the radialplane of the enclosure on the outside thereof.

In an embodiment of the invention, the oxidizer injection pipe forms anangle of inclination β formed by the longitudinal axis of the pipe andthe line passing through the fuel injection point and tangential to thetrajectory of the gas circulation after the injection point, said angleβ ranging between 5° and 80°.

According to an embodiment of the invention, the oxidizer injectionmeans consist of at least two low-angle pipes arranged in opposition onthe enclosure, a first pipe allowing injection at the top of theenclosure and a second pipe allowing injection at the bottom of theenclosure.

In an embodiment of the invention, the oxidizer injection pipes form anangle of inclination β′ defined with respect to the longitudinal axis ofthe pipe and the radial plane of the enclosure, ranging between 5° and80°.

In the combustion chamber according to the invention, the withdrawalmeans forms an angle γ defined with respect to the longitudinal axis ofthe withdrawal means and the radius of the circle formed by theenclosure, and oriented in the direction of circulation of the fumes.

According to an embodiment of the invention, angle γ ranges between 20°and 85°.

According to an embodiment of the invention, the tube has a sectionwhose size ranges between 100 mm and 2000 mm.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will be clear fromreading the description hereafter, with reference to the accompanyingfigures given by way of example, wherein:

FIG. 1 diagrammatically shows a cross-sectional view of a variant of thedevice according to the invention,

FIG. 2 diagrammatically shows a side view of a variant of the deviceaccording to the invention,

FIG. 3 diagrammatically shows a longitudinal cross-section of thevariant of FIG. 2 of the device according to the invention, and

FIG. 4 diagrammatically shows a side view of a second variant accordingto the invention.

DETAILED DESCRIPTION

As illustrated in FIG. 1, the combustion chamber according to theinvention comprises an enclosure (1) having the shape of a bent tubeforming a closed circle or a closed oval. What is referred to as a tubeis an element of elongate shape, hollow, of any section. The section ofthe tube can notably be circular (9), oval (FIGS. 2 and 3) or polygonal,preferably triangular (9′) (FIG. 4), but also square or right-angled.When the section of the bent tube forming the enclosure is circular (9),one speaks of a toric chamber.

The enclosure can also be defined as a hollow volume generated by thedisplacement of any generating surface in the direction of a closedcurve representing the locus of its barycenters.

Dimension d of the section of the tube (diameter or side for example)forming the enclosure ranges between 100 mm and 2000 mm.

The walls of the combustion chamber are made of a specific alloy such asHaynes 230©, Kanthal APM©, MA956© or HR120©, or any other material ofthe same type. Externally, these walls can be coated with materialsallowing the reactor to be cooled from the outside. The outlettemperature of the chamber is thus adjusted and the walls are protectedfrom the hot spots generated in the chamber. In cases where the chamberis not cooled by means of a dedicated device, an inner refractorizationof the chamber can also be considered. The materials used forrefractorization are, for example, ceramics or refractory cements, orany other material of the same type.

The combustion chamber comprises fuel injection means (2) and oxidizerinjection means.

When the fuel is liquid, the fuel injection means is a nozzle, and thisnozzle is advantageously provided with an internal ensuring mixing ofthe fuel with a spraying fluid.

When the fuel is a gas, such as natural gas for example, the injectionnozzle is a high-velocity nozzle allowing to reach rates preferablyabove 100 m/s, for example a commercial injection nozzle of RegeMAT©type used by WS.

The present invention is of course not limited to these two types offuel and it also encompasses the use of solid fuels. In this case, theinjection nozzle can consist of a discharge valve wherein said fuel iscarried by a fluid such as vapour for example.

The combustion chamber comprises oxidizer injection means (3), saidoxidizer being, within the scope of the invention, either a gas with avery high oxygen concentration, usually above 90%, or pure oxygen.

These oxidizer injection means can be an injection nozzle, preferablytubular and made of a refractory material.

Oxidizer injection can be assisted by any means, such as recycled fumes,which affords the advantage of accelerating the oxidizer injection ratewhile limiting the concentration heterogeneities due to the injection ofoxygen.

It is also possible to assist the injection of oxidizer by steam, whichallows to reduce the formation of solid unburnt materials such as sootfor example.

Typically, the oxidizer is injected with a high impulse, which allows afast fume circulation to be maintained.

In any case, the injection means are understood to be pipes, i.e. tubesof circular, oval or polygonal shape in the rest of the description.

One of the features of the device according to the invention is thatinjection and withdrawal are carried out so as to limit the appearanceof hot spots. The fuel and the oxidizer are therefore injectedseparately (thus without premixing) onto the outside axis of theenclosure and the angle of incidence of the pipes allowing injection isoptimized so as to avoid any hot gas impact against the walls.

Positioning the injection means also requires considering the pipelocation constraints.

For injection of the fuel according to a first variant illustrated inFIG. 1, an injection nozzle is arranged on the outside of the circleformed by the enclosure, i.e. in the radial plane (P) of the enclosure.Angle α characterizes the inclination of the pipe and it is preferablydefined as the angle formed by longitudinal axis (20) of the pipe andline (21) passing through the injection point and tangential to mediancircular axis (A) of the gas circulation trajectory (4) after theinjection point.

However, constructing a combustion chamber according to the inventionrequires welding several elements and, in some cases, it is thereforenot possible for injections to be provided exactly on the outside.According to a second variant of the invention, the injection means thusconsist of a system of pairs of low-angle pipes (2′, 2″) in oppositionwith respect to radial plane (P), as illustrated in FIGS. 2, 3 and 4.These pipes are arranged so as to allow injection at the top and at thebottom of the enclosure, considering the section of the tube. Angle α′of these pipes is defined with respect to the longitudinal axis of pipes(20′, 20″) and radial plane (P) of enclosure (1).

These angles α and α′ are constrained by the construction limits and byspray angle (8). Angle α corresponds to the inclination required tocompensate for the entrainment and keep a centered injection in theenclosure, it ranges between 5° and 80°, preferably between 15° and 50°.Angle α′ ranges between 5° and 80°.

The nozzles having a very low flexibility on the flow rates and airstarting requiring a wide injection amplitude, at least one injectionpoint is specified, but it is possible to claim several ones in series.The number of pipes used is thus adjusted so as to gain in flexibilityon the flow rates without drastically modifying the flow rates pernozzle. The number of pipes ranges between 1 and 15, preferably between2 and 10. Besides, this device improves the fuel distribution, whichallows to improve the combustion quality and to avoid formation of hotspots.

For the injection of oxidizer and notably oxygen (or air for starting),pipe (3) used is arranged on the outside of the circle formed byenclosure (1), i.e. in the radial plane of the enclosure and inclinedalong an angle β to ensure a circulation after mixing that is in linewith the enclosure. This angle characterizes the inclination of the pipeand it is defined as the angle formed by longitudinal axis (30) of thepipe and line (31) passing through the injection point and tangential tomedian circular axis (A) of gas circulation trajectory (4) after theinjection point. It ranges between 5° and 80°, preferably between 15°and 45°. In case of setting problems, in the same way as for fuelinjection, it is possible to use an injection with 2 pipes arranged inopposition. These pipes are arranged so as to allow injection at the topand at the bottom of the enclosure. In this case, the pipes form each anangle β′ (not shown) defined with respect to the longitudinal axis ofthe pipe and the radial plane of enclosure (1). This angle has to beminimal to maximize the induced entrainment, it ranges between 5° and80°.

Angle θ formed by each fuel and oxidizer injection position with thecentre of the enclosure, i.e. the angle formed by lines (22, 32) passingthrough the fuel and oxidizer injection points and centre (C) of theenclosure, must range between 10° and 150°, preferably between 15° and90°.

In a preferred embodiment of the invention, the richness defined as thequotient of the ratio of the flow rates of fuel/oxidizer in operationand of the fuel/oxidizer ratio under stoichiometric conditions rangesbetween 0.5 and 3.

The combustion chamber also comprises a means (5) for withdrawing thecombustion fumes. This withdrawal means is arranged in a place where itdoes not disturb the recirculated gas circulation (4). Withdrawal means(5) thus is arranged inside the circle formed by enclosure (1) so as toachieve low-angle withdrawal. Longitudinal axis (51) of the withdrawalpipe therefore forms an angle γ with radius (r) of the circle formed atthe outlet point of the withdrawal means. This withdrawal pipe isdirected in the direction of circulation of the fumes and angle γadvantageously ranges between 20° and 85°. The withdrawal trajectorythus runs on from fumes circulation (4). The withdrawal pipe has adiameter S ranging between 10 mm and 250 mm.

During operation of the combustion chamber, a large flow of hotcombustion fumes circulates and recirculates permanently in the loop ofthe toroid. In this case, one speaks of internal recycle in contrastwith the external recycle techniques (that may also be considered foroxycombustion). This large flow of fumes is sustained by means of highinjection rates in the chamber. Consequently, as they enter the chamber,the oxidizer and fuel jets undergo a strong swirl and a high dilution bythe fumes. Swirling provides contacting of the reactants underconditions as diluted and as homogeneous as possible, as well asspreading of the reaction zone over the entire chamber. These fumes aresufficiently hot to cause auto-ignition of the reactants.

The fuel is first injected and swirled within the hot fumes. These fumescontain a small proportion of residual oxygen. The route of the mixture,from the fuel injection point to the oxidizer (air or O₂) injection,therefore allows the gas to mix, to dilute and to partly react.

The oxidizer is then injected with a high impulse. This impulse allowsto sustain fast circulation of the fumes and additional swirling.Combustion continues throughout the travel through the loop.

Part of these fumes is extracted in a zone where the fumes flow is notdisturbed.

The temperature and the composition of the fumes are substantiallyhomogeneous in the entire enclosure. This temperature ranges, undernominal operating conditions, between 600° C. and 2000° C., preferablybetween 800° C. and 1500° C., so as to limit NOx formation linked withpossible parasitic air inflows or with the nitrogen of the oxidizer.

The high air/O₂ injection rate ranging between 20 m/s and 500 m/s,preferably between 100 m/s and 250 m/s, sustains a high entrainment ofthe gases present in the combustion chamber. This high recirculationfavours swirling and dilution of the species present so as to achieve acombustion as homogeneously distributed as possible in the volume of thechamber.

The consequences of this operation type are as follows:

combustion takes place in two stages

strong swirling is obtained at the injection points

the high circulation rate of the hot fumes spreads the combustion overthe entire volume of the toroid and sustains the combustion.

A device that prevents formation of hot spots and provides a volumecombustion over the entire combustion chamber is thus achieved.

Furthermore, the combustion being distributed over the entire enclosureand not concentrated in form of a flame at the level of the injectionpoints, the temperature never exceeds 2000° C. and the hot spots arelocated at the centre of the toroid. The combined effects of wallcooling from the outside, of the absence of direct hot gas impacts andof the combustion homogeneity allow to obtain wall temperatures below1000° C.

This layout (enclosure geometry, separate injections, centralwithdrawal) favours recirculation of the combustion fumes andhomogeneous combustion over the entire chamber volume.

The imagined concept is besides particularly suited and interesting forlow powers. In fact, in this power range, the geometry of the chamberaccording to the invention allows to ensure good recirculation over asmall volume, whereas the use of pure oxygen causes, on usual geometrieswith low powers, an entrainment that is generally too low. This gasrecirculation is attributable to the centrifugal force that minimizesthe ratio of the outlet gas flow rate to the recirculated gas flow rate.

Besides, preheating due to the recirculation of the hot fumes allows towiden the operating flexibility and to cover, for example, a widerichness range.

Furthermore, with small geometries, the high surface area-to-volumeratio facilitates the possible cooling of the chamber and therefore thecombustion quality control via control of the mean temperature of thechamber.

The possibility of lowering the temperature of the hearth down torelatively low temperatures (unlike air combustion chambers) withoutstopping the combustion allows to jointly consider, with suitablerichnesses, other applications such as partial oxidation, production ofhydrogen or of oxygen compounds such as methanol.

It must be clear to the person skilled in the art that the presentinvention should not be limited to the details given above and that itallows embodiments in many other specific forms without departing fromthe field of application of the invention. The present embodimentsshould therefore be considered by way of illustration and they can bemodified without however departing from the scope defined by theaccompanying claims.

1) A combustion chamber comprising an enclosure having at least one fuelinjection means, at least one oxidizer injection means and at least onecombustion fumes withdrawal means, wherein the enclosure has the shapeof a bent closed tube of any section, and the fuel and oxidizerinjection means are arranged on the enclosure so as to be offset by anangle θ formed by each of the oxidizer and fuel injection positions withrespect to centre of the enclosure, ranging between 10° and 90°, andwherein the oxidizer injection means is a means of injecting an oxidantthat is a gas with an oxygen concentration above 90%. 2) A combustionchamber as claimed in claim 1, wherein the enclosure has the shape of atube bent in a closed circle. 3) A combustion chamber as claimed inclaim 1, wherein the enclosure has the shape of a tube bent in an oval.4) A combustion chamber as claimed in claim 1, wherein the section ofthe tube is circular, oval or polygonal. 5) A combustion chamber asclaimed in claim 1, wherein the section of the tube is triangular. 6) Acombustion chamber as claimed in claim 1, wherein the withdrawal meansis arranged within the circle formed by the enclosure so as to achievelow-angle withdrawal. 7) A combustion chamber as claimed in claim 1,wherein the fuel injection means comprises at least one injection pipearranged in the radial plane of the enclosure on the outside thereof. 8)A combustion chamber as claimed in claim 7, wherein the fuel injectionpipe forms an angle of inclination α formed by a longitudinal axis ofthe pipe and line passing through the fuel injection point andtangential to median circular axis of gas circulation trajectory afterthe injection point, said angle α ranging between 5° and 80°. 9) Acombustion chamber as claimed in claim 1, wherein the fuel injectionmeans comprises at least two low-angle pipes arranged in opposition onthe enclosure, a first pipe allowing injection at the top of theenclosure and a second pipe allowing injection at the bottom of theenclosure. 10) A combustion chamber as claimed in claim 9, wherein thefuel injection pipes form an angle of inclination α′ defined withrespect to a longitudinal axis of the pipe and the radial plane of theenclosure, ranging between 5° and 80°. 11) A combustion chamber asclaimed in claim 1, wherein the oxidizer injection means comprises atleast one injection pipe arranged in the radial plane of the enclosureon the outside thereof. 12) A combustion chamber as claimed in claim 11,wherein the oxidizer injection pipe forms an angle of inclination βformed by a longitudinal axis of the pipe and line passing through thefuel injection point and tangential to median circular axis of gascirculation trajectory after the injection point, said angle β rangingbetween 5° and 80°. 13) A combustion chamber as claimed in claim 1,wherein the oxidizer injection means comprises at least two low-anglepipes arranged in opposition on the enclosure, a first pipe allowinginjection at the top of the enclosure and a second pipe allowinginjection at the bottom of the enclosure. 14) A combustion chamber asclaimed in claim 13, wherein the oxidizer injection pipes form an angleof inclination β′ defined with respect to the longitudinal axis of thepipe and the radial plane of enclosure, ranging between 5° and 80°. 15)A combustion chamber as claimed in claim 1, wherein the withdrawal meansforms an angle γ defined with respect to the longitudinal axis of thewithdrawal means and radius of the circle formed by the enclosure, andoriented in the direction of circulation of the fumes. 16) A combustionchamber as claimed in claim 15, wherein angle γ ranges between 20° and85°. 17) A combustion chamber as claimed in claim 1, wherein the tubehas a section whose size ranges between 100 mm and 2000 mm.